Multi-layer piezoelectric actuators 53 constituted from piezoelectric layers and electrodes stacked alternately one on another have been known as an example of the multi-layer piezoelectric element. The multi-layer piezoelectric actuators 53 can be divided into two categories: fired-at-once type and stacked type where piezoelectric porcelain and internal electrode sheet are stacked one on another alternately. When the requirements to reduce the voltage and manufacturing cost are taken into consideration, the multi-layer piezoelectric actuator 53 of fired-at-once type is more advantageous for the reason of smaller layer thickness and higher durability.
FIG. 9 shows a multi-layer capacitor as a typical multi-layer electronic component of the prior art, which is constituted from a dielectric material 21 and internal electrodes 22 stacked alternately. The internal electrode 22 is not formed over the entire principal surface of the dielectric material 21, but is formed in a so-called partial electrode structure. The internal electrode 22 having the partial electrode structure is placed in every other layer in a staggered manner, so that the internal electrodes 22 are connected to external electrodes 23, that are formed on the side faces of the multi-layer electronic component, alternately in every other layer (refer, for example, to Patent Document 1).
FIG. 8A shows a multi-layer piezoelectric element of the prior art, which is constituted from piezoelectric layers 11 and internal electrodes 12 stacked alternately one on another. As shown in FIG. 8A and FIG. 8B, the internal electrode 12 is not formed over the entire principal surface of the piezoelectric layer 11, but is formed in a so-called partial electrode structure. The internal electrodes 12 having the partial electrode structure are stacked in a staggered manner, so that the internal electrodes 12 are connected to external electrodes 15, that are formed on the side faces of the multi-layer electronic component, alternately in every other layer. Fundamental structure of the multi-layer piezoelectric element is the same as that of the multi-layer capacitor shown in FIG. 9, and is manufactured by printing a paste for the internal electrode in a predetermined pattern on a ceramic green sheet stacking a plurality of the green sheets having the paste for the internal electrode printed thereon, and firing the stack (refer, for example, to Patent Document 2).
The multi-layer piezoelectric element comprises a column-shaped stack 13 formed by stacking the piezoelectric layers 11 and the internal electrodes 12 alternately. Placed on both ends in the direction of stacking are inactive layers 14. The internal electrodes 12 are formed so as to be electrically connected to the external electrode 15 at the end on the right hand side in one layer and at the left hand side in the next layer. When the multi-layer piezoelectric element is used as the multi-layer piezoelectric actuator, the external electrodes 15 are further provided with lead wires fastened thereto by soldering.
The internal electrode is formed from an alloy of silver and palladium and, in order to fire the piezoelectric material and the internal electrode at the same time, composition of metals contained in the internal electrode is set to 70% by weight of silver and 30% by weight of palladium (refer to, for example, Patent Document 3).
This composition has been employed because the composition of 70% by weight of silver and 30% by weight of palladium corresponds to a temperature of 11500° C. on solidus and a temperature of 1220° C. on liquidus, and a piezoelectric material consisting mainly of perovskite type oxide such as PbZrO3—PbTiO3 can be sintered optimally a temperature around 1200° C.
The internal electrode is made of the metal composition that contains silver-palladium alloy instead of 100% silver because, when a voltage is applied between the pair of opposing internal electrodes that are made of silver without palladium content, the so-called silver migration occurs in which silver atoms of the pair of internal electrodes propagate along the device surface from the positive electrode to the negative electrode. Silver migration occurs particularly conspicuously in an atmosphere of high temperature and high humidity.
In recent years, since it is required to make a compact multi-layer piezoelectric actuators capable of achieving a large amount of displacement under a high pressure, continuous operation is carried out over a long period of time with a higher electric field applied.
Patent document 1: Japanese Unexamined Utility Model Publication (Kokai) No. 60-99522
Patent document 2: Japanese Unexamined Patent Publication (Kokai) No. 61-133715
Patent document 3: Japanese Unexamined Utility Model Publication (Kokai) No. 1-130568